Drilling well bores for oil and gas exploration and recovery is known in the industry. During the initial formation of the well bore and even subsequent to the formation of the well bore, it is often desirable to perform various down-hole operations in the well. However, it can be difficult to readily control and carry out many of those down-hole operations due to the fact that the operator is located at the surface while the operations are being carried out several thousands of feet below the surface. For example, difficulties may arise with respect to mechanically actuating various parts of the tool located in the well bore. Similarly, it may be difficult to determine when the mechanical actuation has been completed. Also, controlling fluid flow to the tool parts may present certain problems for an operator located at the surface.
To illustrate some of the problems associated with down-hole tools, the construction and operation of one type of down-hole tool, an underreamer, will be discussed. Underreamers are used in the oil and gas industry to enlarge or drill-out the diameter of the well bore at any point along its length. Enlargement of the well bore may be necessary, for example, to provide space for cementing a liner in the bottom of the well. To effect drill-out of the well bore, the underreamer is inserted into the hole and the cutter arms which form a part of the underreamer are then extended outwardly while the underreamer is rotated. The rotating cutter arms contact and cut away the wall of the well bore and thereby enlarge the size of the well bore.
One problem associated with conventional underreamers is that fluid circulation through the tool cannot be carried out without also exerting a hydraulic pressure that extends the cutter arms to the extended or underreaming position. Consequently, fluid circulation and cutter arm extension cannot be performed independently of one another.
Another problem associated with conventional underreamers is the difficulty that arises in maintaining the cutter arms in the extended position during underreaming operations. If the cutter arms cannot be maintained at the extended position, the effectiveness of the underreamer is reduced and the underreaming operation, if successful, takes an unnecessarily long amount of time.
Conventional underreamers are also problematic in that the operator at the surface has no positive indication when the cutter arms have reached their fully extended position. The operator's inability to accurately determine when the cutter arms have reached their fully extended position means either that the underreamer must be removed from the well bore in order to determine the extent of the well bore enlargement or alternatively, the underreamer must be operated longer than is necessary to ensure that the cutter arms have been fully extended. In either case, valuable time is lost.
A further concern in the construction and operation of an underreamer involves the drilling efficiency of the cutting surfaces. If the drill cuttings are not removed from the cutting surfaces during the cutting operation, the drilling efficiency of the underreamer will be adversely affected.